1. Field of the Invention
The present invention relates to an idle speed control of an engine of an automobile, and more particularly, to an idle speed control of an engine of an automobile equipped with an automatic transmission including hydraulically actuated friction engaging means.
2. Description of the Prior Art
The idle speed of an engine of an automobile is controlled, as a matter of principle, to be an allowable minimum speed level, on the condition that the stable operation of the engine is ensured, in order to save the fuel consumption. Since such a minimum speed level to ensure the stable operation of the engine needs to be elevated when atmospheric air and/or the engine is cold, it is known, as the art of "idle-up", to increase the idle speed according to air temperature and/or engine temperature, particularly when the engine is started from the cold state. The idle-up is generally also executed in an automobile equipped with an air conditioner so that the operation of the air conditioner is maintained during the engine idling condition of an automobile.
According to the developments of the modern electronic automotive control systems various arts have been proposed to control the idle speed of the engine in relation to other operational conditions of the automobile, such as to prepare two staged idle-up speed levels for an engine combined with an automatic transmission so that a lower speed idle-up is executed during an idling where the automatic transmission is shifted to one of the drive ranges or the reverse range with no brakes being actuated, while a higher speed idle-up is executed during an idling where the automatic transmission is shifted to the neutral or parking range or a brake is actuated, from the view point of maintaining a stable idle operation of the engine while avoiding an unintentional start of the automobile (Japanese Utility Model Laid-open Publication 1-111160);
to control the idle operation of an engine combined with a torque converter under a modification of a target value for the idle speed according to whether a torque converter traction control system which selectively disengages the torque converter during the idling is in operation or not, so that the target value is increased to ensure the stable operation of the engine when the torque converter is disengaged by the traction control system as compared with the conventional idling in which the engine, although it rotates at a relatively low speed while idly driving the torque converter, is supplied with a correspondingly increased rate of fuel for generating more power for idly driving the torque converter than in an idling with no such torque converter load, and therefore operates stably even at such a low idle speed (Japanese Patent Laid-open Publication 61-250353);
to control the idle operation of an engine in relation to oil temperature of an automatic transmission so that the ignition timing of the engine is delayed when the oil temperature of the transmission rises, thereby moderating the peaks of the engine output torque in the idling so as thereby to obtain the effect of decreasing a gear noise of the transmission which increases as the oil temperature of the transmission rises due to a reduction of viscosity of oil in the backlash spaces in the meshing gears (Japanese Patent Laid-open Publication 61-167170), and
to control the idle operation of an engine combined with an automatic transmission in relation to shifting into operation of the transmission so that, when the range select lever is shifted from the parking or neutral range to one of the drive ranges or the reverse range, an additional amount of fuel is supplied to the engine with a delay time which is shorter as oil temperature of the transmission is higher, so as thereby to improve the stability of the idle operation of the engine in a period between the setting up of a speed stage in the transmission and a depression of the accelerator pedal, wherein the delay time is controlled to be a minimum time span not to allow the engine operation to become unstable due to an increase of the load imposed on the engine as a result of the engagement of the transmission but not to cause revving up of the engine by too early increase of fuel supply, taking it into consideration that the oil temperature of the transmission governs the speed of setting up speed stages of the transmission (Japanese Patent Laid-open Publication 62-131939).
During the idling of an automobile equipped with an automatic transmission a substantial part of the power generated by the engine is consumed by an oil pump which provides a hydraulic pressure source for the operation of the automatic transmission. In the oil pumps practically available for such a purpose a certain amount of leakage of oil from its delivery side to its suction side is unavoidable. The amount of such oil leakage generally increases as the oil temperature rises, because the viscosity of oil lowers according to the elevation of its temperature. In view of this fact it has conventionally been a tacit understanding in the design of the oil pump that its capacity is so selected that it can provide a required amount of delivery of oil even at a relatively high oil temperature estimated to occur in a wide range of operating conditions, in spite of a correspondingly increased leakage of oil. This fact means that in a lot of operating conditions the oil pump is delivering a substantial amount of needless oil, consuming a corresponding amount of power for no use.
Therefore, there is another aspect in the elevation of oil temperature of the automatic transmission which would be a subject for further improvement with respect to the idle operation of the engine of an automobile equipped with an automatic transmission.
In this connection, it is noted that the largest delivery of oil from the oil pump is required during a speed stage shifting of the transmission. In other words, when the transmission is operating at any speed stage with certain hydraulically actuated friction engaging means such as clutches and brakes being maintained in their fully engaged condition, no substantial amount of delivery of oil is required. Therefore, it is only during the speed stage shifting that the problem of shortage of delivery of oil from the oil pump would occur due to an increased leakage of oil in the oil pump according to the elevation of oil temperature.